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United States Patent |
6,025,681
|
Yu
,   et al.
|
February 15, 2000
|
Dielectric supported radio-frequency cavities
Abstract
A device which improves the electrical and thermomechanical performance of
an RF cavity, for example, in a disk-loaded accelerating structure. A
washer made of polycrystalline diamond is brazed in the middle to a copper
disk washer and at the outer edge to the plane wave transformer tank wall,
thus dissipating heat from the copper disk to the outer tank wall while at
the same time providing strong mechanical support to the metal disk. The
washer structure eliminates the longitudinal connecting rods and cooling
channels used in the currently available cavities, and as a result
minimizes problems such as shunt impedance degradation and field
distortion in the plane wave transformer, and mechanical deflection and
uneven cooling of the disk assembly.
Inventors:
|
Yu; David U. L. (Rancho Palos Verdes, CA);
Lee; Terry G. (Cupertino, CA)
|
Assignee:
|
Duly Research Inc. (Rancho Palos Verdes, CA)
|
Appl. No.:
|
017138 |
Filed:
|
February 2, 1998 |
Current U.S. Class: |
315/500; 315/505; 315/506 |
Intern'l Class: |
H05H 009/00; H05H 009/02 |
Field of Search: |
315/500,505,506,5.39,5.41,5.42
313/62,359.1,360.1
250/396 R
|
References Cited
U.S. Patent Documents
4181894 | Jan., 1980 | Pottier | 315/500.
|
4350921 | Sep., 1982 | Liska et al. | 313/360.
|
4733132 | Mar., 1988 | Miyata et al. | 315/5.
|
4906896 | Mar., 1990 | Swenson | 315/5.
|
5049753 | Sep., 1991 | Flesner | 250/396.
|
5523659 | Jun., 1996 | Swenson | 315/506.
|
Primary Examiner: Westin; Edward P.
Assistant Examiner: Wells; Nikita
Attorney, Agent or Firm: Keschner; Irving
Goverment Interests
GOVERNMENTAL RIGHTS IN INVENTION
This invention was made with Governmental support under Small Business
Innovation Research (SBIR) Contract No. DE-FG03-96 ER 82156 awarded by the
Department of Energy to DULY Research Inc. The Government has certain
rights in the invention.
Parent Case Text
RELATED APPLICATIONS
This application is based on U.S. Provisional Application No. 60/036,907
filed Feb. 5, 1997.
Claims
What is claimed is:
1. An RF cavity for accelerating charged particles introduced therein
comprising:
a housing structure having an inner surface and a longitudinal axis;
a plurality of metallic members positioned within said structure and space
along said longitudinal axis, each metallic member having inner and outer
edges; and
supporting members for supporting each metallic member within said
structure, said supporting members being fabricated from a high strength
dielectric material characterized by high thermal conductivity, low
electrical conductivity, high threshold of electrical breakdown voltage,
low dielectric constant and low dielectric loss, wherein said dielectric
material is made of polycrystalline diamond.
2. The RF cavity of claim 1 wherein each metallic member comprises a disk
having a hole formed at the center thereof.
3. The RF cavity of claim 1 wherein each supporting member has inner and
outer edges, the inner edge thereof being connected to the outer edge of
an adjacent metallic member.
4. The RF cavity of claim 3 wherein the outer edge of each of said
supporting members is connected to the inner surface of said housing
structure.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to radio-frequency (RF) structures, such
as klystron or accelerator cavities wherein a special dielectric is used
to support metallic elements inside the structure whereby superior
electrical and thermomechanical performances are obtained.
2. Description of Prior Art
Present-day radio frequency (RF) cavities such as those in linear
accelerators for charged particle beams typically comprise a series of
metallic disks with a beam hole at the center. The metallic disks are
separated by a distance equal or close to an integral fraction (1/N, where
N is an integer) of the wave length of the pulsed RF power which energizes
the beam. The RF frequency ranges widely from hundreds of megahertz to a
tens of gigahertz and higher. In common practice, the inter-disk distance
is maintained by a metallic spacer between two adjacent disks as in the
case of a traveling wave (TW) structure, or by two or more metallic rods
as in the case of a "plane wave transformer (PWT)" structure. A
conventional PWT accelerating structure 10 is shown in FIG. 2. In this
structure, the disks 12, having irises 13 formed therein, are connected by
four rods 14 which also provide circulating coolant to the interior of the
disks 12 to take away the heat generated by the coupling of the
iris-loaded metallic disks to the RF field. The supporting rods 14 are
connected to end flanges (not shown). The accelerating structure, or disk
assembly, 10, is placed inside a cylindrical tank 16 which includes an
input port for the incoming RF power. A unique feature of the PWT
structure is that there is a very strong and efficient RF coupling between
the electromagnetic field inside the outer tank 16 and all the
accelerating cells between the disks 12. Another advantage of the PWT is
that as a result of the strong coupling, the mechanical tolerance of the
disks assembly is quite loose, thus making fabrication easier and less
costly. On the other hand, there are several disadvantages of the PWT
design shown in FIG. 2. The metallic rods 14 connecting the disks 12
distort the accelerating electromagnetic field, thus degrading the beam
quality. The coupling of the magnetic field into metallic rods increases
loss and reduces the shunt impedance and the quality factor of the
accelerating cavity, thus lowering the efficiency. The method of heat
removal in the metal disk by circulating coolant in its interior requires
complicated design and expensive brazing of two halves of each disk.
Finally, since the rods 14 are only supported at the two end flanges,
excessive deflection at the center of a long disk assembly can only be
avoided by increasing the diameter or wall thickness of the connecting
rods, which would compound the electrical problems.
SUMMARY OF PRESENT INVENTION
The present invention provides a device which improves the electrical and
thermomechanical performance of an RF cavity, for example, in a
disk-loaded PWT accelerating structure.
A washer (typically less than one millimeter thick) made of polycrystalline
diamond, or a material having similar thermal, dielectric and mechanical
properties, is brazed in the middle to a copper disk washer, and at the
outer edge to the PWT tank wall, thus efficiently dissipating heat from
the copper disk to the tank, while providing strong mechanical support to
the metal disk. The washer structure eliminates the longitudinal
connecting rods and cooling channels of the conventional design, and as a
result minimizes such problems as shunt impedance degradation and field
distortion in the PWT, and mechanical deflection and uneven cooling of the
disk assembly.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the present invention as well as other
objects and further features thereof, reference is made to the following
description which is to be read in conjunction with the accompanying
drawings wherein:
FIG. 1 is schematic diagram of the present invention as incorporated in a
plane wave transformer accelerating structure;
FIG. 2 is a schematic diagram of a prior art plane wave transformer
accelerating structure;
FIG. 3 is a diagram showing the brazing joints of the dielectric support to
the metallic disk and to the outer tank wall;
FIG. 4 is a diagram showing a tool to be used for precision alignment
during brazing;
FIGS. 5A and 5B show an alternate method of connecting the dielectric
support to the outer tank wall; and
FIGS. 6A and 6B illustrate the dielectric-disk assembly before insertion
into the tank shown in FIG. 5A.
DESCRIPTION OF THE INVENTION
The present invention provides apparatus for holding individually and
collectively a set of metallic partitions, or disks to form a plurality of
cavities in an RF structure. A preferred embodiment of this invention is
in an RF linear accelerator for a charged particle beam (such as
electrons, protons, or ions). In the case of a cylindrical (rectangular)
accelerating structure, each of these partitions is a circular
(rectangular) flat disk with a circular (rectangular) hole at its center
through which the beam passes. The disks are held in place in a prescribed
manner by circular (rectangular) slabs made of special high strength
dielectric material characterized by high thermal conductivity, low
electrical conductivity, high threshold of electrical breakdown voltage,
low dielectric constant and low dielectric loss. An example of such
material is polycrystalline diamond, which is commercially available.
Typical chemical, electrical and thermomechanical properties of the
polycrystalline diamond are shown below:
Thermal conductivity >1300 w/m-.degree.K
Electrical resistivity >10.sup.11 ohm-cm
Thermal expansion coef. Approx. 2.times.10.sup.-6 .degree.C.,
25-200.degree. C.
Dielectric constant 5.7
Loss tangent <0.0005 @ 15 GHz
Dielectric strength 100-300 MV/m
Chemical behavior Insert to acids, alkalis and solvents
Oxidation behavior Resistant to 700.degree. C.
The dielectric material (polycrystalline diamond) can be precision cut with
a laser. A commercially available form of polycrystalline diamond is
Diamonex.RTM., manufactures by Diamonex Inc., Allentown, Pa.
FIG. 1 shows a preferred embodiment of the invention as utilized in a plane
wave transformer photoelectron accelerator (PWTPA) 20. In the PWTPA 20, a
pulsed laser beam hitting a cathode produces bunches of photoelectrons
which are accelerated synchronously by a standing-wave Rf field in phase
with the advancing electrons along the axis 32 of the PWTPA cavity 20. A
cavity is formed in the space between two adjacent metal (e.g. copper)
disks 22. A beam hole 24 is provided at the center of each metal disk 22.
In accordance with the teachings of the present invention, each disk 22 is
supported by a thin, concentric, dielectric washer, or slab, 26 (typically
less than one millimeter thick) with the inner edge of the washer brazed
to the outer edge of the metal disk 22. The dielectric washer 26 provides
the only necessary support for each metal disk 22. Each dielectric washer
26 is preferably made of material similar to that whose properties are
shown in the above table. The outer edge of the dielectric washer is
brazed at a prescribed axial location to the wall of a metallic (e.g
copper plated steel) housing tank 28 of the PWTPA 20 thus securing the
required position of each metal disk 22. RF power from an external source
(e.g. a klystron) is coupled to the PWTPA through an inlet port 30. A
plane-wave like, TEM mode of electromagnetic wave is established in the
annular region between the inner tank and the disk assembly. Housing tank
28 is coupled to the end flanges 34 and 36 and a water jacket 38 is
provided to cool the PWTPA 20. A vacuum port 40 is provided to maintain a
vacuum within PWTPA 20. A TM mode of electromagnetic wave which provides
the accelerating gradient is established along the axis 32 of the
cavities. Since there is no cavity wall expect the faces of the metal
disks 22, the fields in the tank 28 and those in the cavities are strongly
coupled. The edges of the dielectric washer 26 are metalized. The
locations of the brazing joints of the dielectric washer 26 to the metal
disks 22 and the housing tank 28, illustrated by reference numerals 42 and
44, respectively, are shown in FIG. 3.
In case the thermal expansion coefficient of the dielectric washer 26 (as
in polycrystalline diamond) is lower than that of the metal disk 22 (such
as copper), the disk may be made of base material, such as molybdenum,
with a thermal expansion coefficient close to that of the dielectric, and
coated with metal (e.g. copper). This minimizes thermal stress and avoids
tearing of the brazing joints during thermal transients. Alternatively,
the dielectric washer 26 can first be brazed to thin concentric copper
cylinders and subsequently to a thick inner copper disk and the outer
wall.
A carbon fixture 50 in the shape shown in FIG. 4 may be used to align the
dielectric washer 26 with the metal disk 22 and the outer tank wall 28.
Fixture 50 is used to hold the work during brazing of a dielectric washer
26 to thin concentric copper cylinders 51 and 53. Items 54 are braze
rings. Alignment screws 55 are provided for adjustment purposes. Since
carbon has a lower thermal coefficient of expansion than copper, pressure
is maintained between the dielectric washer and cooper during the braze
cycle. After completion of the brazing of dielectric washer 26 to thin
copper cylinders 51 and 53, fixture 50 is removed. The separate tank
sections with dielectric washers and a metal disk already in place are
then brazed together to form an integral cylindrical tank.
There are several advantages of supporting the metal disks with a
dielectric washer in the manner described above.
In the present invention as illustrated in FIG. 1, the heat generated by RF
power impinging on the surfaces of metal disks 22 is removed by conduction
through the dielectric washer 26 to the outer tank 28. The high thermal
conductivity of the dielectric (e.g. polycrystalline diamond) ensures
efficient removal of the heat without the necessity of any active cooling
of the disks. There is no need to have any coolant channels inside the
metal disks, thus simplifying the design.
Further, the direct dissipation of heat is achieved by conduction from the
metal disks 22 through the dielectric washer 26 to the outer tank wall 28.
The temperature of the outer wall 28 can be controlled by a heater or a
constant temperature water bath or jacket 38. This in turn controls the
temperature of the irises 24 at the metal disks 22 and makes frequency
tuning an easy task.
In addition, there is no metal rods or cups connecting the metal disks (as
in the conventional PWT design shown in FIG. 2). The absence of such
perturbing metal elements in the PWTPA of the present invention alleviates
distortion of electromagnetic field symmetry and reduces loss due to RF
interaction with such metal elements. Thus, the embodiment of the present
invention in the PWTPA 20 results in a higher quality factor Q and a
higher shunt impedance R.
Further, each metal disk 22 is mechanically supported by a dielectric
washer 26 with high compressive strength. Compared with the conventional
PWT design in which the disks are connected by a set of longitudinal rods
which are supported by end flanges, the embodiment of the present
invention avoids mechanical deflection, or "sagging" of the disk assembly,
ensuring improved beam dynamics.
Finally, each metal disk 22 is a solid piece. There is no need to braze two
halves of each disk in order to create cooling channels in the disk
interior. This reduces the cost manufacturing.
An alternative method of attaching a brazed metal disk/dielectric washer to
the outer tank wall 28' is shown in FIGS. 5A and 5B. In this method, a
slip fit instead of brazing joint is used at the dielectric washer/tank
wall junction. A series of circumferential female retainers 60 are first
machined into the inside of the tank wall 28'. Each set of retainers 60,
at given axial location, consists of two narrow grooves, each spanning an
arc of 90.degree. around the tank wall circumference, and spaced
180.degree. apart from the other. The grooves 60 are precisely spaced
apart on the inner tank wall along the axial direction to correspond to
the required spacing between the metal disks of the PWTPA 20. The outer
edge of the dielectric washer 26 is machined into two shallow fins 29 (see
FIG. 6B), each spanning an arc of 90.degree. around the outer
circumference of the dielectric washer and spaced 180.degree. apart from
the other. The depth of each fin on the dielectric washer 26 is the same
as the depth of the groove 60 on the wall of tank 28'. The thickness of
each fin is same as the width of groove 60.
A metal disk is brazed to the center of a dielectric washer as described
earlier. To insert a dielectric washer (with a metal disk already brazed
at its center) into the retaining grooves 60 on the wall 28' the fins on
the dielectric washer 26 are first aligned at exactly the same axial
location of grooves 60 on the tank wall, but 90.degree. apart
circumferentially from the grooves. The dielectric washer/metal disk is
then rotated by 90.degree. to fit the fins into grooves 60. Sufficient
radial, axial and circumferential clearances are provided so that the fins
on the dielectric washer 26 can be tightly fit into grooves 60 on the tank
wall without interfering with the insertion process. This alternate method
of attaching the washer/disk to the outer tank 28' has the advantage of
ease of insertion and replacement of the disk assembly but at the expense
of somewhat degraded electrical and thermal performance of the PWTPA 20.
The retaining grooves 60 on the outer wall of tank 28' would induce some
loss. In addition, the slip fit between the dielectric washer 26 and the
retaining grooves 60 would increase the thermal resistance at the joint,
particularly during down cycles of thermal transients. During up cycles of
thermal transients, however, the temperature of the dielectric washer 26
is higher than that of tank wall 28'. A slight interference fit may
result, thus mitigating the reduction of thermal resistance at the
slip-fit junctions.
FIGS. 6A and 6B shows the dielectric-disk assembly before insertion into
the tank 28'. Shown is copper retainer 70, tank wall 28', fins 29,
dielectric disk, or washers 26, copper disk 72 and iris, or beam hole, 74.
While the invention has been described with a reference to its preferred
embodiments, it will be understood by those skilled in the art that
various changes may be made and equivalents may be substituted for
elements thereof without departing from the true spirit and scope of the
invention. In addition, modifications may be made to adapt a particular
situation or material to the teachings of the invention without departing
from its essential teachings.
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